1. Field of the Invention
The present invention relates to telecommunications cable closure assemblies. More particularly, the present invention relates to fiber optic cable closures for enclosing fiber optic cable splices.
2. Description of Related Art
Typically, large diameter telephone distribution cables (e.g., approximately 3" in diameter) may be used to carry hundreds of pairs of electrically conductive wires, such as copper wires, from a central office location to distribution points. Where two ends of such a cable are spliced together, the large cable is cut open, and pairs of wires are spliced together. The splice area is ordinarily housed within a protective cover known as a splice closure. The spliced cable and closure may be suspended in the air from a suspension strand with the total length of cable between suspension poles being 100-200 feet in length. The closure must restrain each end of the cable with an axial force sufficient to overcome the forces created by repeated longitudinal expansion and contraction of the cable due to natural temperature changes. An end of the cable which is not adequately restrained by the closure may simply pull itself out of the closure due to the thermal expansion and contraction of the cable involved. At the same time, the point of entry of the cable into the closure must be hermetically sealed to protect the exposed wires within the closure from the external environment.
Electrically conductive cables are typically restrained within a closure by a heavy duty hose clamp which is secured around the end of the cable, within the closure, and secured to the end plate of the closure with a strain relief bracket. The hose clamp puts a tremendous amount of radial force onto the cable, but holds the large cable securely without interfering with the electrical transmission of the signal. A hermetic seal is formed between the cable and the cable entrance port in the end plate by liberally applying a mastic material to the cable entrance port of the two halves of the end plate and securing the two halves around the cable. As the end plate bolts are tightened, the mastic flows between the cable and the end plate to form a seal which protects the interior of the closure from the external environment.
In recent years, communication via optical fiber cable has enjoyed a rapid rate of growth. Optical fibers provide the ability to transmit large quantities of information by light impulses and thus promise to increase in use in the future. Optical fiber cables are constructed in many configurations, all of which typically include three functional elements, an outer sheath which surrounds the optical fibers, a strength member to withstand cable tension during either placement or when permanently installed, and multiple very fine gauge optical fibers. In most cases the fine gauge optical fibers are organized and given additional protection in buffer tubes within the sheath.
More specifically, the advantages of signal transmission over optical fibers includes increased capacity and the elimination of undesirable interference and cross-talk which may be present with conventional electrically conductive wires. The increased capacity results in fewer optical fibers needed in each cable as compared to typical electrically conductive cables. In addition, each optical fiber is significantly smaller in diameter than an electrically conductive wire. For these reasons, a typical fiber optic cable has a fraction of the diameter of a typical electrically conductive cable.
A disadvantage of the use of optical fibers is that light signals transmitted by optical fiber may be adversely impacted by excessive bending of the optical fiber. Excessive bending can significantly reduce the efficiency of light transmission as it travels along the optical fiber. Therefore, an optical fiber cable must be treated with more deference as compared to electrically conductive cable to prevent excessive radial bending and radial pressure on the optical fibers.
One example of a fiber optic cable splice closure includes multiple fiber optic cable entrance ports in either end of the fiber optic cable closure. The fiber optic cable sheath is restrained within the closure by wrapping the cable sheath with a pressure sensitive adhesive coated rubber tape and securing the wrapped cable with a hose clamp which is in turn secured to a strain relief bracket within the closure. The fiber optic cable strength member is secured within the closure by an anchor. A hermetic seal is provided between the cable and the cable entrance port by wrapping the fiber optic cable with a sealing tape formed of a mastic material and capturing the wrapped cable in the entrance port as the closure is sealed.
The fiber optic closure as described does not provide an adapter to restrain and seal fiber optic cables within a large diameter cable entrance port typically provided with electrically conductive cable closures. In addition, the fiber optic cable restraint system relies solely on a hose clamp and corresponding strain relief bracket for cable sheath retention. As a result, a tremendous amount of radial force is required in order to maintain the fiber optic cable sheath in place as the sheath expands and contracts due to changing temperature conditions. Conversely, if the radial force provided by the hose clamp is not applied to its maximum potential, the sheath could potentially pull out of the hose clamp and the cable entrance port, exposing the optical fibers to the environment and risking disruption of service.
An enormous amount of capital has been invested in the existing telecommunications infrastructure which is designed to accommodate electrically conductive cables. As systems transition to higher capacity, smaller diameter fiber optic cables, it is desirable to adapt the existing infrastructure, including closures, etc., for use with smaller diameter fiber optic cables.
Accordingly, a need presently exists for an improved fiber optic cable splice closure for sealing fiber optic cable splices and effectively restraining the fiber optic cable sheath while effecting a minimum amount of radial force on the optical fibers and which provides a hermetic seal between the optical fiber cable and the cable entrance port. Further, a need presently exists for a means for adapting existing closures designed for use with large diameter electrically conductive cables for use with smaller and more bending sensitive fiber optic cables.